A standard cylinder-head gasket is compressed between the cylinder head and the block of an engine and is formed with through going holes for the cylinders, the head-mounting studs, and the coolant and lubricant circuits. This gasket is normally formed as a perforated plate of steel covered with a durable and heat-resistant material such as asbestos, although in some cases use is made of glass fibers, or a synthetic resin such as Kevlar. For a given engine there are normally three thicknesses available for installation normally when the engine is rebuilt.
In today's engines it is standard, particularly in diesels, to have a very high compression ratio, typically around 20:1 (ratio of maximum to minimum cylinder volume) as compared to the older low-compression engines where the ratio is 9:1 to 10:1. As a result of this high compression it is extremely important that the cylinder-head gasket be dimensioned to very close tolerances. This is particularly the case for a rebuilt engine.
Not only is asbestos a particularly difficult and dangerous material to work with, but the precise requirements of such a gasket, in several different thicknesses, make production complex and expensive.